1. Field of the Invention:
The present invention relates to an apparatus and method for use in the installation of an offshore platform structure above a previously-installed substructure.
2. Description of the Prior Art:
As the exploration for oil and gas reserves has progressed into offshore waters, platforms capable of supporting the required drilling and production equipment have been installed above promising petroleum reservoir locations.
Each platform is usually installed in two or more separate sections. The first section consists of a partially submerged substructure supported by the ocean floor. A portion of this substructure extends above the surface of the water. A platform structure which forms the upper second section is then placed upon the substructure. The platform structure usually has an array of four to eight downwardly-depending legs which mate with and are welded to a corresponding array of four to eight upwardly-directed leg supports which form the upper portion of the substructure.
The platform structure is placed upon the substructure by a derrick barge. This barge carries a crane of sufficient weight-lifting capacity to lift the platform structure from an equipment supply vessel, position the platform structure over the substructure, and then lower the structure downward until the legs of the structure contact the leg supports of the substructure.
As can be imagined, even minor five to six foot ocean swells that periodically impact the derrick barge can cause the platform structure, when suspended by cables from the crane, to swing back and forth similar to a pendulum. Upward and downward movement of the derrick barge will also cause similar movements in the suspended platform. The final mating alignment between the legs and leg supports is usually very difficult, with damaged mating members not uncommon as the platform structure pounds up and down on the support legs, until the weight of the platform structure is fully transferred to the substructure.
As can be imagined, failure of the crane as the platform structure is suspended in the air could cause catastrophic loss of the platform structure with attendant risk to human life on the platform substructure, equipment supply vessel, and derrick barge.
There are economic disadvantages as well in the use of such a derrick barge, due to the cost of its operation. In some undeveloped areas of the world, the use of such a derrick barge would be economically unfeasible due to the high cost associated with the single trip of the barge to a country having minimum petroleum reservoir development.
Accordingly, it is desirable to present a method and apparatus for installation of a platform structure without the damage, danger and expense associated with use of a derrick barge.
Additionally, after the platform structure has been placed upon the substructure an inordinate amount of time usually must be spent in welding the legs of the platform structure to the substructure's leg supports, due to misalignments which exist between the legs and leg supports. Accordingly, it is also desirable to present a method and apparatus that allows final mating adjustments to be made between the legs and leg supports prior to welding the legs and leg supports together.